build: remove bootstrap dependency on poetry

[cpcloud]

This PR removes a dependency on poetry that gets put into most dependencies
as a buildInputs if poetry2nix isn't bootstrapping, which if I'm reading the
code correctly is nearly everywhere (__isBootstrap is false by default).

This is a problem when building using crossSystem (I'm trying to
build a Docker image using musl) because any dependency that overlapped with
poetry's dependencies would cause both the build and host platform versions
of poetry's deps to be installed, causing a conflict during build.

It also appears that this bootstrap dependency is not necessary, at least
according to the current test suite. The only package that needed modification
was pendulum.

[cpcloud]

Here's a CI run where I'm building a musl-based image: https://github.com/cpcloud/protoletariat/runs/4576549106?check_suite_focus=true

[cpcloud]

@adisbladis Thoughts on this PR?

[adisbladis]

This will break some downstreams (not all of the cases where this is required is covered by tests).
I would be open to something like !__isBootstrap && !isCross so that we can solve your issue while not breaking various projects using this code.

[cpcloud]

Can you write down such a scenario? I'm happy to write one or more test cases, so that folks don't spend a ton of time tracking down the problem to this (like I did) without a test breaking.

[adisbladis]

This is really the same as any other build-system missing caused by python-poetry/poetry#2789.

Adding poetry to each build was just a hack to circumvent that breakage for stuff depending on Poetry which I figured would be a fairly cheap workaround considering you are very likely to have poetry in your dependency graph anyway if you use this tool.

Maybe as a minimal reproduction case you could try an environment with https://github.com/tweag/pynixutil in it?

[adisbladis]

I've decided to merge this since the only downside is that we need a bit more overrides in the stdlib, but it's not possible to fix the cross use case via overrides.